1. Field of the Invention
The present invention relates to a fuel reforming apparatus which is employed, for example, for generating hydrogen necessary for portable fuel cells for general power supply or necessary for fuel cells to be mounted in electric automobiles and converts alcohol material or hydrocarbon material to hydrogen-rich, reformed gases.
2. Description of the Related Art
In the conventional fuel reforming apparatus of this kind, there is, for example, the isothermal reactor module of BPS (Ballard Power Systems Inc., Canada) which is suitable for methanol steam reforming reaction, disclosed in Japanese Patent Laid-Open No. 6-507145. FIG. 20 is a rough schematic view of a fuel reforming apparatus provided with an isothermal reactor module. In the figure, 50 is an isothermal reactor module, 51 a heat recovery module, 52 a methanol tank, 53 a water tank, 54 a heat-transfer fluid reservoir, 55 a heat-transfer fluid heater, 58 a reforming reaction passage, 58' a reformed-gas passage, 59 a methanol evaporation passage, 59' a methanol liquid heating passage, 60 a water evaporation passage, 60' a water liquid heating passage, 61 a heat-transfer fluid passage, 62 a supplied-fuel burner, and 63 a steam superheating passage.
The operation of the fuel reforming apparatus of this type will now be described. The heat recovery module 51 supplies methanol and water from the tanks 52 and 53 to the methanol liquid heating passage 59' and the water liquid heating passage 60' with pumps P1 and P2, respectively. The methanol and water are preheated with the reformed-gas passage 58 by the high-temperature reformed gases that come from the isothermal reactor module 50.
The preheated methanol and water are guided to the methanol evaporation passage 59 and the water evaporation passage 60 of the isothermal reactor module 50, in which they are heated and evaporated by the heat-transfer fluid passing through the heat-transfer fluid passage 61. The heat-transfer fluid is sent from the heat-transfer fluid reservoir 54 to the heat-transfer heater 55. In the heat-transfer heater 55, the heat-transfer fluid is heated by the supplied-fuel burner 62.
After being heated, the heat-transfer fluid is sent to the isothermal reactor module 50 and used as a heat source for evaporation and reforming. Thereafter, heat-transfer fluid is returned to the reservoir 54 and again is circulated to the heat-transfer fluid heater 55 by a pump P3. Methanol and water are mixed together after evaporated in the methanol evaporation passage 59 and the water evaporation passage 60, and the mixture is supplied to the steam superheating passage 63, in which it is heated up to an optimum temperature of reforming reaction. The heat necessary for evaporation and heating is supplied by the heat-transfer fluids passing through the heat-transfer fluid passages 61 provided on both sides of the methanol evaporation passage 59, the water evaporation passage 60 and the steam superheating passage 63.
Thereafter, steam comprising the heated methanol and water is supplied to the reforming reaction passage 58 of the isothermal reactor module 50, and by using an appropriate reforming catalyst (e.g., catalyst of Cu-Zn), the endothermic reaction of methanol steam (CH3OH+H2O.fwdarw.3H2+CO2) is performed. The heat necessary for the reforming reaction is supplied by the heat-transfer fluids passing through the heat-transfer fluid passages 61 provided on both sides of the reforming reaction passage 58.
Thereafter, the reformed gases are sent to the heat recovery module 51, and the gases are cooled down to a suitable temperature (usually 100.degree. C. or less) for being supplied to fuel cells, by the heat exchange between the reformed gases and methanol-water.
A plan view of this isothermal rector module 50 is shown in FIG. 21, a plan view of a heat-transfer fluid plate is shown in FIG. 22, and a partial exploded side view is shown in FIG. 23. In the figures, 61 is a heat-transfer fluid passage, 61' a heat-transfer fluid passage plate, 31 a sealing sheet, 27 a heat-transfer fin, 66 a baffle plate, 67 a housing (shown by hatching in the figures), and 33 a reforming catalyst. The isothermal reactor module 51 is constituted by the sealing sheet 31, baffle plate 66 with heat-transfer fins 65, housing 67 enclosing substantially the circumference of a heat-transfer surface, and heat-transfer fluid passage plate 61' having the heat-transfer fluid passage 61 interiorly.
As shown in FIG. 22, an array of parallel passage grooves provided in the interior of the heat-transfer fluid plate 61' are arranged with a pattern in which outgoing passages and incoming passages alternately appear, and the grooves form the heat-transfer fluid passage, thereby averaging the temperature distribution within the surface of the heat-transfer fluid plate. The heat of the heat-transfer fluid is transferred through the heat-transfer fins 27 to the reforming catalyst 33, whereby the heat necessary for reforming reaction is supplied.
The baffle plate 66 fills the heat-transfer fins 27 with the reforming catalysts 33, also forms a meandering or labyrinthine passage, and allows the reformed gases to pass through the passage. As shown in the plan view of FIG. 21, the heat-transfer fins 27 form a plurality of concentric, arcuate walls on the baffle plate 66. This increases the velocity of the reformed gases at a predetermined space speed, and consequently, the thickness of the boundary layer along the wall of the passage is reduced, the heat transfer from the wall to the reformed gases is accelerated, and the methanol reforming rate is enhanced.
In the case where reformed gases are generated by methanol steam reforming reaction and a solid polymer type fuel cell is operated, the reformed gases contain a large amount of carbon monoxide (CO) which poisons the electrode catalyst of the fuel cell and therefore reduces the cell life, so there is a need to reduce the concentration of the CO in the reformed gases to about 10 ppm.
As the conventional fuel reforming apparatus of this kind, a method and apparatus for selectively oxidizing the carbon monoxide present in a hydrogen-containing gas mixture, proposed by the aforementioned BPS incorporation, is disclosed in Japanese Patent Laid-Open No. 7-502205. In this method of oxidizing carbon monoxide (CO), oxygen or an oxygen-containing gas mixture is introduced at locations on the way to the latter portion of the isothermal reactor, and CO is oxidized to carbon dioxide, thereby reducing CO concentration. A cross sectional view of the selective oxidizing reactor of this invention is shown in FIG. 24. In the figure, 69 is a fin block, 70 an air distribution plate, 72 a reformed gas inlet portion, 73 a reformed gas outlet portion, and 74 a plurality of secondary air inlet portions.
In this oxidization method of CO, a primary amount of oxygen-containing gases (for example, air) are premixed into reformed gases (gas mixture containing hydrogen, CO2, and CO) and are guided to the inlet 72 of the selective oxidizing reactor. Then, the reformed gases are contacted with the catalyst in the selective oxidizing reactor to promote the oxidization of CO, and a further amount of oxygen or an oxygen-containing mixture is introduced into the reaction chamber through a plurality of secondary air inlet portion.
The selective oxidizing reactor is constituted by the fin block 69 contacting the air distribution plate 70. The fin block 69 has a plurality of heat-transfer fins extending from the bottom portion to the direction of the air distribution plate, and the heat-transfer fins are joined with the air distribution plate to form a meandering or labyrinthine channel therebetween.
The bottom portion of the fin block 69 includes heat-transfer fluid passages 61 for supplying heat to the interior of the selective oxidizing reactor or removing heat from the selective oxidizing reactor. The carbon monoxide (CO) concentration distribution of the selective oxidizing reactor obtained by this oxidization method of CO is shown in FIG. 25. For the CO concentration distribution without secondary air, the carbon monoxide (CO) concentration increases significantly at some point along the passage through the reactor, as indicated by a solid line in FIG. 25.
It is believed that this increase is due to the effect of reverse shift reaction (H2+CO2.fwdarw.CO+H2O). The addition of secondary air along the pathway significantly reduces the CO concentration (as indicated by a two-dotted line in FIG. 25), and a low CO concentration of about 10 ppm is shown at the outlet.
In another conventional fuel reforming apparatus, there is, for example, a fuel reforming apparatus which suppresses the CO concentration in reformed gases to a relatively lower level, as disclosed in Japanese Patent Laid-Open No. 7-126001. The structure of this fuel reforming apparatus is shown in FIG. 26. In this apparatus, an evaporation portion 75, a reforming-shifting portion 76, and an oxidizing-removing portion 77 are made as stacked structures of an evaporating layer 78 and a heating layer 79, a reforming-catalyzing layer 80 and a heating layer 81, and an oxidizing-catalyzing layer 82 and a heating layer 83, respectively, and are integrally connected along with a combustion portion 84.
This structure is capable of reducing the size of the apparatus compared with the aforementioned conventional apparatuses, also suppressing the thermal loss of the heat-source gases generated in the combustion portion 84, and making efficient use of heat. In the reforming-shifting portion 76, the CO concentration in reformed gases is suppressed, and even at the oxidizing-removing portion 77, the remaining CO concentration is reduced down to approximately 100 ppm of the CO concentration level that is supplied to fuel cells.
The CO concentration control means of the reforming-shifting portion 76 controls the amount of the heat-source gases from the combustion portion, which is introduced into the reforming-shifting portion 76, in correspondence with the CO concentration in reformed gases, and varies the temperature distribution within the reforming-catalyzing layer to vary the ratio of a reforming reaction region and a shift-reaction region, thereby adjusting the CO concentration of reformed gases.
Also, as described in Japanese Patent Laid-Open No. 7-335238, a stacked plate type fuel reforming apparatus has been provided as another conventional fuel reforming apparatus. This apparatus is provided with a reforming reaction portion, a partial oxidization-reaction portion, and a catalytic combustion portion. In this apparatus, the stacked structure of the fuel reforming apparatus, the operation method relative to load variation when the apparatus is started and operated, the vaporization method of liquid fuel, the integral structure between the reforming apparatus and a fuel cell, the mixing method of fuel and air, and the flow adjusting means of the material gas that is supplied to the reforming portion are described.
In addition, to improve the workability and producibility of the apparatus, the elements forming the apparatus are constituted by flat plate elements employing heat-transfer fins. Furthermore, as described in Japanese Patent Laid-Open No. 5-319801, in the name of a stacked type methanol reformer, the structure of a methanol reformer with reforming cell units and heating cell units alternately stacked is proposed as still another conventional fuel reforming apparatus.
However, in the isothermal reactor module of the conventional fuel reforming apparatus disclosed in Japanese Patent Laid-Open No. 6-507145, since the isothermal reactor module is heated indirectly by heat-transfer fluid, attached devices, such as a reservoir, pumps, and a heat-transfer fluid heater, and piping become necessary and therefore there arises the problem that the construction of the entire system becomes complicated.
Also, although the baffle plate, the seal sheet, and the heat-transfer fins (a plurality of arcuate walls) have been provided in the interior of the reactor to increase the linear velocity of reformed gases and to increase the heat transfer from the walls to the reformed gases, there arise the problems that the structure and configuration of the members are complicated and workability suitable for mass production is unobtainable.
In addition, in the selective oxidizing method and apparatus of the carbon monoxide (CO) in reformed gases disclosed in Japanese Patent Laid-Open No. 7-502205, while secondary air has been introduced at locations on the way to the latter portion of the isothermal reactor to promote the oxidization of carbon monoxide (CO), a plurality of secondary air inlet portions such as those shown in FIG. 24 are needed due to the introduction of secondary air and therefore flow control devices of the same number as the inlet portions become necessary.
Additionally, since the isothermal reactor is provided separately from the fuel reforming apparatus and operated under isothermal conditions, heating fluid and cooling fluid become necessary, so there arise the problems that the entire system is complicated and structure suitable for mass production is unobtainable.
Moreover, in the fuel reforming apparatus disclosed in Japanese Patent Laid-Open No. 7-126001, the carbon monoxide (CO) concentration in reformed gases is about 100 ppm and therefore is still in a high-concentration level in order to operate a solid polymer type fuel cell, so that a further reduction in the carbon monoxide (CO) concentration becomes necessary.
While the CO concentration control means has controlled the amount of the heat-source gases from the combustion portion which are introduced into the reforming-shifting portion 76 in correspondence with the CO concentration in reformed gases, the amount of the heat-source gases that flow to the combustion portion is determined by the heat necessary for reforming reaction and necessary for vaporization of liquid feed, and cannot be varied arbitrarily. Consequently, it is difficult to independently control the amount of the heat-source gases with this structure.
In addition, oxidization of carbon monoxide (CO) is exothermic reaction, and in order to maintain the inlet temperature of the CO oxidization portion at a temperature (250.degree. C. or less) suitable for CO oxidization, a portion which becomes an endothermic source of 200.degree. C. or less becomes necessary.
Moreover, in the fuel reforming apparatuses disclosed in Japanese Patent Laid-Open No. 5-319801 and Japanese Patent Laid-Open No. 7-335238, apparatus miniaturization is achieved, but since any means has not been taken of a CO reduction in the reformed gases, these apparatuses are insufficient as a fuel reforming apparatus that is employed in solid polymer type fuel cells.